II. THERMOMETRY. 221 



one or two centimetres above the bottom of the jar. When the apparatus has 

 been placed in the large beaker -jar, water or oil is poured into the latter, so 

 that the tube is quite immersed. The jar is then heated, agitating the fluid 

 meanwhile with a moving apparatus. 



The millimetre scale serves to determine the height of the fluid in the great 

 jar, together with the difference between the position of the mercury in the 

 little tube and that on the outside of it. With respect to determining this 

 difference, the pressure of the fluid in the great beaker-jar and the barometric 

 height must be taken into consideration. Thus, when the pressure of the fluid 

 in the great beaker-jar is equal to half a centimetre of mercury, and the baro- 

 metric height 76^ centimetres, the thermometer must be observed, when the 

 mercury in the tube is 1 centimetre higher than that at the outside of it. 



To know whether a fluid is homogeneous, two experiments must be made, 

 one with a fluid which is partially evaporated. In both cases the results 

 must be the same. 



When the vapour of the fluid in the tube has the pressure of one atmosphere 

 the boiling point of the fluid must be observed. 



1003. Photographs of Old Thermometers ; a small al- 

 cohol thermometer, with Florentine scale, and four larger ones by 

 Michelo du Crest (1754). 



Professor Hagenbach-Bischoff, Director, The Physical 

 Institute in the Bernoullianum, Basle. 



In these alcohol thermometers zero indicates the temperature of the cellar 

 under the Observatory of Paris, and 100 the boiling point of water. 



1004. Trough for comparing Thermometers, provided 

 with in- and out-flow tubes for water, and stirring apparatus. 



Dr. J. W. Gunning^ Professor of Chemistry at the " Athe- 

 nceum illustre" Amsterdam. 



The thermometers are placed in a loose frame, in which they may be trans- 

 ported from one trough into another containing water of another temperature. 

 Two loose sheets of glass, placed on either side of the frame, prevent the 

 inner portion of the water from cooling. 



1005. Dial Thermometer, designed by L. R. Briilme, Leyden. 

 Dr. D. de Loos, Director of the Secondary Town School, 



Leyden. 



This thermometer is intended to admit of a large number of students seeing, 

 from a distance, the change in the volume of the mercury as the temperature 

 varies. 



In the mercury is a small glass tube, balanced by another similar tube, both 

 being joined together by a thread, which is suspended over a small copper box, 

 at the extremity of which is a needle moving over a screen (or dial). 



When the mercury expands, the first glass tube rises, and the needle moves. 

 In the other case the tube descends. 



1006. Mercurial Dial Thermometer, adapted for class 

 experiments on specific and latent heat. 



Prof. W. F. Barrett, Dublin. 



The expansion of the mercury in the bulb of the thermometer lifts a small 

 iron piston which communicates its motion to the index hand. Small varia- 



